Multimode fiber's information carrying capacity typically can be described in terms of a bandwidth length product (measured in units of MHz-km or GHz-km) that can be used to determine how far a system can operate at a given bit (data) rate. The data rate for multimode fiber and VCSEL-based short distance communications have increased from gigabit to 10 Gb/s, 16 Gb/s and 25 Gb/s or higher. The (absolute) bandwidth of a given multimode fiber with a specific length can be measured. The scaled bandwidth as determined by the bandwidth length product can be determined by multiplying the measured bandwidth with the length of multimode fiber measured to have the unit of GHz·km or MHz·km.
Presently, the main measurement method for measuring the bandwidth of multimode optical fibers is the differential mode delay (DMD) method. The measurement is done in the time domain by measuring the output of the pulse launched from a single mode fiber at the input end of the multimode fiber using various offsets of the single mode fiber relative to the multimode fiber core. The transfer function for various launch conditions can be determined from the DMD measurement and the bandwidth of the multimode fiber can be calculated.
Another method of measuring the bandwidth that is less widely used is the frequency-sweeping (FS) method. Both the DMD and FS methods provide a measurement of bandwidth for a single wavelength. In the FS method, a light source (e.g., laser diode) is used to inject light with narrow line width (e.g., 0.1 nm or even 0.05 nm) of a select amount of power into a test fiber. The laser is modulated from a low frequency (for an approximately zero reference level) to a high frequency (in excess of the 3 dB bandwidth). The relative output optical power is recorded as a function of the frequency. The input modulated signal launched into the test fiber is determined in a similar manner by measuring the output on a shorter reference length of fiber. The input and output powers as a function of frequency are then used to calculate the bandwidth.
One way to determine the bandwidth is to find the frequency for which the received signal power has dropped by 3 dB relative to the signal power at a low frequency. Such bandwidth is also often referred to as the “3 dB bandwidth.” The bandwidth obtained from the FS method depends upon the launch conditions.
There is interest in using long-wavelength VCSELs in combination with multimode fiber. Multimode VCSEL based laser sources emit light at a wavelength between 840 nm and 860 nm at various data rates such as 1 Gb/s, 4.25/s, 10 Gb/s, 16 Gb/s and 25 Gb/s. Multimode optical fibers are also used with LEDs at wavelengths around 1300 nm and at relatively low data rates. The bandwidth of a multimode fiber is wavelength dependent. The wavelength dependency is determined by the material used for making the fiber and the refractive index profile of the fiber. To have a large bandwidth at around 850 nm, it is desirable the peak bandwidth of the multimode fiber to be centered at or near 850 nm. In addition, multimode optical fibers can be used at different wavelengths.
Consequently, it is important to know the bandwidth of multimode fibers at different wavelengths. Furthermore, detailed characterization of a multimode optical can be accomplished by using bandwidth data obtained at different wavelengths. To this end, it would be advantageous to have methods and apparatus that allow for quick and accurate bandwidth measurements of multimode optical fibers at different wavelengths.